The Problem and the Proposed Solution

Energy supply represents one of the top priorities of the present global economy with important social implications. In this context, gas transmission systems represent fundamental infrastructures moving large volumes of natural resources among different Countries and rising a number of national and international security issues as energy reprisals are frequent consequences of geo-political conflicts.

Long-term exploited gas pipelines experience corrosion-induced fracture combined with aging. Often, the transported gas serves as a source of hydrogenation of the metal wall with the effect of progressive bulk material degradation under combined electrochemical and mechanical action. These phenomena increase the risk of uncontrolled brittle failure, with potentially significant economic losses and severe environmental consequences.

Material aging is practically unavoidable but the related safety issues can be managed and the risk of failure can be prevented by the implementation of effective measures for the diagnosis and preservation in safe service conditions of pipelines. This action requires the definition of reliable prediction models of the evolution with time of the degradation phenomena induced by the interaction with the environment.

Safety margins with respect to brittle failure can be evaluated starting from the quantification of the variation of the electrochemical properties, depending on number factors including the material composition and the environmental conditions, and by a complementary approach based on indentation tests, providing a continuous monitoring of the current mechanical state.

The present project aims at providing non-destructive diagnostic tools for the safety assessment of exercised pipelines for gas transportation in the presence of hydrogen-induced degradation, not yet included in normative documents.

Short Description of the Project

Natural gas transmission systems represent important energy infrastructures running all over the World. In some regions or Countries like Ukraine, they play an international strategic role. Still, the majority of gas pipelines in Ukraine is close to the planned service life or even beyond the depletion time. Thus, their preservation in safe serviceability conditions requires that reliable monitoring and diagnostic tools are developed and implemented in order to prevent failures, which may result in significant economic losses and severe environmental consequences. On the other hand, similar safety requirements are shared by all operated pipeline networks.

Recent research results show that the quite frequent exposure of the metal wall to hydrogen transported with raw gas mixtures reduces the material toughness and increases the risk of brittle fracture and stress corrosion cracking. It was also found that an important factor for the loss of serviceability of main pipelines during their long-term exploitation is the bulk material degradation with time, an issue not properly taken into account in engineering calculations at the design stage. On the other hand, hydrogen-induced degradation phenomena are not yet included in normative documents.

It is worth mentioning that similar questions on the stability with time of the initial properties of metals are posed by the observed trend in recent years toward the use of high-strength steels, particularly sensible to aggressive environments, in the construction of new transmission lines.

The continuous monitoring of the current status of pipelines represents a fundamental tool for their maintenance in safe service conditions. This action can be facilitated by the implementation of non-destructive testing methods resting on static and dynamic indentation. These techniques, which have been used since many years for hardness evaluation, can be applied in situ on the operating component, without the need of extracting material spools and working out specimens.

Indentation tests can be proposed for an effective diagnostic analysis of pipeline steels provided that the geometry of the residual imprint left on the tested material surface is exploited together with a suitable simulation tool of the experiment. The reliability of this approach will be verified in the present project testing laboratory specimens and pipe segments cut from pipelines after long-term service. The mechanical properties determined by indentation and imprint mapping will be compared to the output of standard tensile tests. If successful, the implementation of the diagnostic technique based on indentation tests will produce significant saving in terms of execution times and costs.

The main changes of the electrochemical and mechanical properties of gas pipeline steels during operation will be analysed in the project. This information will be used in combination with a predictive model of hydrogen diffusion and fatigue crack propagation in steel. The envisaged complementary experimental and analytical efforts should produce a reliable prediction method of the material degradation under the combined effect of stresses and absorbed hydrogen and should permit to estimate the likely residual lifetime of environmentally-affected components and to plan the required maintenance interventions.

The most meaningful results gathered from the joint research efforts will be transferred to the end-users UKRTRANSGAZ in Ukraine and eni S.p.A. in Italy. The possible industrial exploitation of the developed methodologies will be facilitated by the contribution of the service company Venezia Tecnologie S.p.A., which will also assist the verification of the non-destructive mechanical testing tool based on indentation.